It is well known to produce semiconductor single crystal material using the Czochralski technique by forming a melt of the crystal material and bringing a "seed crystal" into contact with the melt. The seed is then pulled slowly upwards, the molten material solidifying at the seed-melt interface, thus forming a single crystal billet.
Such a method has been found to be most effective, however, the crystal produced suffers from non-uniform electrical resistivity along its length. This is primarily due to the fact that the doping agents (e.g., arsenic, antimony, gallium or indium) commonly added to the pure semiconductor material (e.g., silicon, germanium), are more soluble in the liquid semiconductor material. Hence, in a growing crystal, the concentration of doping agent in the solid semiconductor crystal is less than the concentration of doping agent in the adjacent liquid melt. Therefore, as a crystal billet is grown by the Czochralski process, a steadily increasing concentration of doping agent is left in the remaining melt resulting in an increase in resistivity along the length of the grown crystal billet.
Although such a crystal billet may be used in many applications where changes in resistivity are not critical, a number of devices, such as transistors, have parameters which vary more or less linearly with the resistivity of the semiconductor single crystal. The difficulty of making devices with predictable characteristics is greatly increased if uniform resistivity is not obtained during the crystal growing process.
A technique used to improve the axial resistivity uniformity of single crystal semiconductor billet is described in U.S. Pat. No. 2,944,875 which issued on July 12, 1960. That patent describes an apparatus in which the concentration of doping agent as well as the volume of the crystal melt is kept substantially constant throughout the seed-pulling process so that a single crystal billet of uniform resistivity may be grown. The apparatus comprises a pair of cylindrical crucibles the second of which is designed to fit loosely into the first and the second or inner crucible is provided with a small hole drilled through the bottom thereof. The crucibles are so arranged that a first charge of high purity undoped or lightly doped semiconductor material is placed in the outer crucible and a second charge of highly doped semiconductor material placed in the inner crucible. Heat is then applied to the double crucible arrangement to form a crystal melt in the inner and outer crucibles. Accordingly, the outer crucible contains a first melt of lightly doped or undoped semiconductor material and the inner crucible, floating within the first melt, holds a second melt of semiconductor material having a substantially greater concentration of dopant therein.
A single crystal billet may be grown from the inner crucible melt by the above-described seed-pulling method and, as the crystal grows, the lightly doped semiconductor material will flow from the outer crucible into the inner crucible, through the hole to maintain the floating inner crucible at an equilibrium level while diluting the dopant concentration of the melt therein. Therefore, until the inner crucible touches the bottom of the outer crucible, the volume of liquid semiconductor material in the inner crucible will remain constant. Since only a small fraction of the dopant agent is used up in the growing crystal billet, the dopant concentration level in the inner crucible melt remains substantially constant during the process which tends to result in the crystal having uniform resistivity along its length.
However, the single opening or hole in the inner floating crucible undesirably permits mechanical mixing of the melt in the two crucibles as well as back dopant diffusion (i.e., diffusion of dopant from the inner to the outer crucible) which alters the dopant concentrations in the inner and outer crucibles and results in non-uniform resistivity of the grown single crystal billet. In order to decrease any back diffusion or mechanical mixing of the inner crucible melt and the outer crucible melt, a narrow channel may be formed between the inner and outer crucibles as shown in an article titled "A Process for Obtaining Single Crystals with Uniform Solute Concentrations," by A. V. Valcic, in Solid State Electronics, 1960, pages 131 to 134.
However, as the solid semiconductor material is simultaneously melted in the crucibles, gas bubbles tend to be captured in such a narrow passageway. The captured bubbles stop the melt flow between the crucibles causing the concentration of the dopant in the inner crucible to rapidly increase requiring that the process be aborted or that the grown billet be used for fabrication of devices having less stringent resistivity standards.